Magnetic Random Access Memory (MRAM) is a non-volatile computer memory technology based on magnetoresistance. One type of MRAM is spin torque transfer MRAM (STT-MRAM), in which a magnetic cell core includes a magnetic tunnel junction (“MTJ”) sub-structure with at least two magnetic regions, for example, a “fixed region” and a “free region,” with a non-magnetic region between. The free region and the fixed region may exhibit magnetic orientations that are either horizontally oriented (“in-plane”) or perpendicularly oriented (“out-of-plane”) relative to the width of the regions. The fixed region includes a magnetic material that has a substantially fixed (e.g., a non-switchable) magnetic orientation. The free region, on the other hand, includes a magnetic material that has a magnetic orientation that may be switched, during operation of the cell, between a “parallel” configuration and an “anti-parallel” configuration. In the parallel configuration, the magnetic orientations of the fixed region and the free region are directed in the same direction (e.g., north and north, east and east, south and south, or west and west, respectively). In the “anti-parallel” configuration, the magnetic orientations of the fixed region and the free region are directed in opposite directions (e.g., north and south, east and west, south and north, or west and east, respectively). In the parallel configuration, the STT-MRAM cell exhibits a lower electrical resistance across the magnetoresistive elements (e.g., the fixed region and free region), defining a “0” logic state of the MRAM cell. In the anti-parallel configuration, the STT-MRAM cell exhibits a higher electrical resistance across the magnetoresistive elements, defining a “1” logic state of the STT-MRAM cell.
Switching of the magnetic orientation of the free region may be accomplished by passing a programming current through the magnetic cell core, including the fixed and free regions. The fixed region polarizes the electron spin of the programming current, and torque is created as the spin-polarized current passes through the core. The spin-polarized electron current exerts the torque on the free region. When the torque of the spin-polarized electron current is greater than a critical switching current density (Jc) of the free region, the direction of the magnetic orientation of the free region is switched. Thus, the programming current can be used to alter the electrical resistance across the magnetic regions. The resulting high or low electrical resistance states across the magnetoresistive elements enable the write and read operations of the MRAM cell. After switching the magnetic orientation of the free region to achieve the one of the parallel configuration and the anti-parallel configuration associated with a desired logic state, the magnetic orientation of the free region is usually desired to be maintained, during a “storage” stage, until the MRAM cell is to be rewritten to a different configuration (i.e., to a different logic state).
Some STT-MRAM cells include dual oxide regions, i.e., another oxide region in addition to an “intermediate oxide region” (which may also be referred to as a “tunnel barrier”) of the MTJ sub-structure. The free region may be between the intermediate oxide region and the another oxide region. The exposure of the free region to two oxide regions may increase the free region's magnetic anisotropy (“MA”) strength as well as lower the damping in the cell core. For example, the oxide regions may be configured to induce surface/interfacial MA with neighboring material of, e.g., the free region. MA is an indication of the directional dependence of a magnetic material's magnetic properties. Therefore, the MA is also an indication of the strength of the material's magnetic orientation and of its resistance to alteration of the magnetic orientation. A magnetic material exhibiting a magnetic orientation with a high MA strength may be less prone to alteration of its magnetic orientation than a magnetic material exhibiting a magnetic orientation with a lower MA strength. Moreover, the low damping, provided by the dual oxide regions, may enable use of a low programming current during programming of the cell. A free region with a high MA strength may be more stable during storage than a free region with a low MA strength, and a cell core with low damping may be more efficiently programmed than a cell core with higher damping.
While the dual oxide regions may increase the MA strength of the free region and lower the damping of the cell core, compared to a free region adjacent to only one oxide region (i.e., the intermediate oxide region), the added amount of oxide material in the magnetic cell core may increase the electrical resistance (e.g., the series resistance) of the core, which lowers the effective magnetoresistance (e.g., tunnel magnetoresistance (“TMR”)) of the cell, compared to a cell core comprising only one oxide region (i.e., the intermediate oxide region). The increased electrical resistance also increases the resistance-area (“RA”) of the cell and may increase the voltage needed to switch the magnetic orientation of the free region during programming. The decreased effective magnetoresistance may degrade performance of the cell, as may the increased RA and programming voltage. Accordingly, forming STT-MRAM cells to have dual oxide regions around the free region, for high MA strength and low damping, without degrading other properties, such as magnetoresistance (e.g., TMR), RA, and programming voltage, has presented challenges.
Other beneficial properties of free regions are often associated with the microstructure of the free regions. These properties include, for example, the cell's TMR. TMR is a ratio of the difference between the cell's electrical resistance in the anti-parallel configuration (Rap) and its resistance in the parallel configuration (Rp) to Rp (i.e., TMR=(Rap−Rp)/Rp). Generally, a free region with a consistent crystal structure (e.g., a bcc (001) crystal structure), having few structural defects in the microstructure of its magnetic material, has a higher TMR than a thin free region with structural defects. A cell with high TMR may have a high read-out signal, which may speed the reading of the MRAM cell during operation. High TMR may accompany high MA and low damping, enabling the use of low programming current.
Efforts have been made to form magnetic material at a desired crystal structure. These efforts include propagating the desired crystal structure to the magnetic material (referred to herein as the “targeted magnetic material”) from a neighboring material (referred to herein as the “crystal seed material”), which propagation may be assisted by annealing the materials. However, simultaneously crystallizing both the crystal seed material and the targeted magnetic material may lead to crystalizing the targeted magnetic material in an undesirable crystal structure before the crystal seed material has a desired crystal structure to fully propagate to the targeted magnetic material. Therefore, efforts have been made to delay crystallization of the targeted magnetic material, until after the crystal seed material is crystallized into a desired crystal structure. These efforts have included incorporating an additive in the targeted magnetic material so the material is amorphous when first formed. The additive may diffuse out of the targeted magnetic material during the anneal, enabling the targeted magnetic material to crystallize under propagation from the crystal seed material, after the crystal seed material has crystallized into the desired crystal structure. However, these efforts do not inhibit the propagation of competing crystal structures from neighboring materials other than the crystal seed material. Moreover, the additive diffusing from the targeted magnetic material may diffuse to regions within the structure where the additive interferes with other characteristics of the structure, e.g., MA strength. Therefore, forming a magnetic material with a desired microstructure, e.g., to enable a high TMR, while not deteriorating other characteristics of the magnetic material or the resulting structure, such as MA strength, can also present challenges.